1. Field of the Invention
The present invention relates to a laser beam scanning optical device adapted for use in a laser beam printer, a copying machine utilizing image formation on a photosensitive member with a laser beam or the like, and more particularly to a light deflector for deflecting a laser beam from a light source.
2. Related Background Art
Such conventional laser beam scanning optical device generally employs a rotary polygon mirror as the light deflector for deflecting a light beam into scanning motion. Such rotary polygon mirror is generally composed of an aluminum alloy, a plastic material or glass, with a reflection increasing film on the reflective faces for light deflection. However, remarkable progress in the mirror surface finishing technology in recent years has put the optical mirror finishing with diamond grinding into practice and has enabled to provide a rotary polygon mirror formed by direct mirror finishing of aluminum or aluminum alloys.
The Japanese Laid-open Patent No. 58-184903 discloses a rotary polygon mirror formed by direct mirror finishing of aluminum or an aluminum alloy followed by formation of a transparent film by anodic oxidation on the finished mirror faces. Such anodized oxide film shows satisfactory adhesion to the underlying aluminum alloy, thus satisfactorily functioning as a protective film for the aluminum mirror face.
However, the anodized aluminum oxide film, disclosed in the above-mentioned Japanese Laid-open Patent No. 58-184903, is practically unusable, due to an excessively large variation in the reflection intensity depending on the incident angle, since the optical film thickness (nd) is designed as m.lambda./2.multidot.cos.theta. (wherein m is a positive integer except zero, .lambda. is wavelength, and .theta. is incident angle) and the reflected light intensity is designed to become maximum in a core of an incident angle at the center of the scanning range.